If you are a Small Business customer, find additional troubleshooting and learning resources at the Support for Small Business site.

You can send outbound SMTP mail from a Microsoft Outlook client but cannot send outbound SMTP mail from a Microsoft Outlook Express client, and the following non-delivery report (NDR) is generated.

The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'user@domain.com'. Subject 'Re: test', Account: 'mail.domain.com', Server: 'mail.domain.com', Protocol: SMTP, Server Response: '550 5.7.1 Unable to relay for user@domain.com', Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC79

NOTE: Do not combine the first option with either the second or third option.

Click to select the My server requires authentication check box, which is located in the Outgoing Mail Server properties in Outlook Express.

On the Exchange Server 5.5 computer, open the Internet Mail Service properties. On the Connections tab, click Accept Connections. Click to clear the Only from hosts using Authentication and the Clients can only submit if homed on this server check boxes.

On the Exchange 2000 server, enable Anonymous Access on the Authentication tab of the SMTP virtual server.